This invention relates to a process for discriminating the color of a surface, and to an apparatus for implementing the process.
More particularly, the invention relates to a process and an apparatus for discriminating the color of a surface by comparison with a previously acquired reference color. The invention further relates to a process for discriminating a range of colors and/or a plurality of colors on a surface.
Throughout this description and the appended claims, the expression "color range" will be used to indicate a plurality of different hues of one color (e.g., for the red color, a range of variation of red hues from light red to deep red).
In object sorting and handling systems, an ability to "recognize" the surface color of the objects may sometimes be useful to facilitate and speed up the object sorting and handling operations. For the purpose, several process for discriminating the color of a surface have been proposed heretofore which all comprise substantially two basic steps, both to be carried out on the same apparatus (also referred to as the "color discriminator" hereinafter), namely: a first "acquisition" step whereby the apparatus is set, manually or automatically, for a reference color, followed by a "recognition" step whereby the color of the object surface being scanned is compared with the previously acquired reference color to check that they are the same, within an optionally adjustable tolerance margin.
More particularly, the acquisition step consists of illuminating with a suitable light source a colored acquisition surface which includes the color to be recognized, and then picking up, onto a light-receiving element, light scattered from said surface so as to generate electric signals which are proportional to the luminous intensity of the picked-up light. These signals are then suitably processed to calculate and store parameters representing the color of the illuminated acquisition surface.
The recognition step consists of using the same apparatus, as previously set during the acquisition step, to illuminate the surface of the object bearing the color to be discriminated and generate electric signals proportional to the luminous intensity of light scattered back from such surface. These signals are then processed to calculate parameters representing the surface color of the illuminated object; these parameters are ultimately compared with those previously stored during the acquisition step to check that they are the same within a predetermined tolerance margin, thereby to recognize the surface color of the illuminated object.
For both the acquisition and recognition steps, the processing of the electric signals generated by the light-responsive element is based on the detection of the three components V.sub.R, V.sub.G, V.sub.B of the three primary colors--red, green and blue--emitted from the three LEDs.
Among the prior color discriminators capable of implementing a process such as that above described, the so-called "solid-state" ones are of particular interest, which additionally to incorporating inexpensive small-size components, can discriminate the color of a surface regardless of the surface distance from the discriminator. Such discriminators employ, as the light sources, three separate emission LEDs, respectively for the red, green and blue colors, and as the light-receiving element, a standard photodiode or phototransistor having a sufficiently wide sensibility in the visible range. In such discriminators, the emission LEDs are supplied by impulse with respective supply currents I.sub.R, I.sub.G, I.sub.B, such that, at each current impulse supplied, corresponding electric signals V.sub.R, V.sub.G, V.sub.B are generated by the photodiode so as that their sum is equal to a predetermined reference value (preferably, the white color). The color is then recognized by measuring the amounts of red, green and blue scattered from the surface of the object being scanned (i.e., measuring the values of the signals V.sub.R, V.sub.G, V.sub.B generated during normal operation of the discriminator).
A color discriminator of the above type, while substantially achieving its objective, still does not allow discrimination of those colors wherein at least one of the color components V.sub.R, V.sub.G, V.sub.B is much smaller than the two others (consider, for instance, the red, green and blue colors themselves, for which the respective color components of that color are much larger than the two others). In this situation, the discriminator above described is little responsive to variations in the other two colors, and therefore, cannot provide accurate discrimination of the different hues of these colors.
Furthermore, there are instances of one component being much smaller than the two others. Here again, the discriminator response to varying hues of that color is poor.